Point defects in crystalline semiconductors and insulators are leading candidates for solid state qubits in quantum technologies [1]} [2]} [3]}, the atomic-like energy structure of levels deep within the band gap providing long-lived coherent states [4]}. Colour centres are a class of point defects that display strong optical transitions, which can allow qubit initialisation and readout, and coherent coupling to an optical network [5]} [6]}. The controlled generation and manipulation of such defects is therefore of great interest.
Many colour centres take the form X-vacancy, where X is a dopant atom, therefore understanding processes by which vacancies can be generated on-demand is paramount to the development of these technologies. Traditional engineering techniques such as ion implantation [7]} and electron irradiation [8]} are successful at producing clusters of vacancies, however, often have limited control over the spatial distribution, particularly in the axial direction. This converts into a stochastic yield of colour centres with moderate positional accuracy following an annealing step. Direct laser writing with single ultra-fast pulses has recently been shown as a promising way to excite isolated Frenkel defects (bound vacancy-interstitial pairs) in diamond at desired locations, such that subsequent annealing can produce high quality nitrogen-vacancy centres [9]}[10]}[11]}[12]}. Similar studies in silicon carbide and gallium nitride reveal that laser writing has the ability to produce a host of different colour centres in wide-bandgap materials [13]}[14]}. Evidence of a lower intensity regime has recently emerged through multi-pulse processing, in which the laser pulse delivers sufficient energy to mobilise the diffusion of vacancies without further Frenkel defect generation or graphitisation [15]} [16]} [17]}. These observations open the question of the degree to which point defects can be engineered using ultra-fast laser techniques, and motivate further study.
